The secondary aluminum industry is concerned with the production of aluminum from slag, dross and scraps of a variety of origins generally unknown. Consequently, the constituents used in the secondary recovery furnaces contain impurities and oxides which must be separated during the melting process in order to segregate the valuable metal. It is important to have a flux which separates the metal from the scrap which includes a variety of dross, slag, recycled scrap, discarded cans and containers, salvaged castings and the like. Because of the nature of some of the scrap including its impurities, oxides and size of individual pieces, it becomes necessary to apply salt fluxes to minimize oxidation and improve slag formation during melting and thereby enhance the recovery rate of the valuable metal.
In particular, the dross type scrap which is produced by the primary aluminum companies and which is made by dry skimming the waste made in remelting furnaces represents one of the most valuable and yet most difficult types of material to process in the secondary aluminum industry.
The primary components of the dross are a plurality of aluminum oxides, hydrated aluminum oxides and entrained metallic aluminum plus small amounts of magnesium chloride, aluminum carbide, aluminum nitrade and small percentages of calcium, silicon, magnesium, iron, zinc and manganese.
Because of the nature of the dross material which normally contains a sizeable fraction of non-metallic materials, several different types of secondary melting furnaces have been developed to process different types of scraps. In general, two types of furnaces are used to process scrap aluminum wastes. These are the rotary furnace and the reverberatory furnace. Both furnaces are suitable for processing all types of aluminum scraps, however, for individual specific scraps, one furnace may have advantages in its ability to recover metallic aluminum from the specific charge material.
Thus, for example, the rotary furnace may be shown to recover higher values of metal from low quality scraps such as drosses when these same drosses are processed by the reverberatory furnace; it does so, however, with the proviso that it is operated less economically because of the need for high quantities of flux salt and because of the inherent problems associated with the production of large quantities of salt containing waste products.
On the other hand, the reverberatory furnace can process the same dross material and produce lower quantities of waste product and recovered metal more economically, but it does so only at the expense of reduced metal recovery.
Thus there exists in the secondary aluminum recovery business a need for a flux or flux and process combination which would provide the highly efficient metal recovery of the rotary furnace while at the same time would produce this high recovery in the reverberatory furnace to take advantage of this type of furnace's economic advantages.
The quantities and compositions of fluxes used in the secondary industry depend very much on the particular plant, the operating practice, and the type of feed material used by numerous companies.
Generally, all rotary type furnaces employ chloride fluxes either as pure NaCl or as a physical mixture of NaCl and KCl. Furthermore, for rotary furnaces, it is customary to charge the salt flux into an empty furnace, melt the salt and then charge the scrap into the molten melt. This is particularly true for mixtures of NaCl and KCl. The advantage of this process is that it provides a flux which has a significant fluidity which acts like water. This fluidity depends on the composition, temperature and concentration ratio of the flux to the inert ingredients in the charge material. In order to take advantage of this fluidity, it is essential that the concentration ratio of flux to inert ingredients be greater than one (1) and that more flux than inert ingredients in the charge be used since too little flux leads to poor metal recovery. Unfortunately, because of the inherent design of reverberatory furnaces which utilize skim eye doors for the effective transfer of thermal energy to open charge wells, the flux mixture, usually 50% NaCl and 50% KCl, cannot be charged and melted on the well first since the liquid runs under and around the skim eye doors where it comes in contact with the high temperature in the combustion box of the furnace and becomes superheated which causes flux loss by evaporation, wastes energy and creates a serious pollution problem.
Furthermore, the flux in a ratio greater than 1 flux to 1 insert oxide cannot be added after the charge has been placed on the reverberatory furnace well because on melting, the same problems exist as if it had been introduced initially.
To overcome this problem, fluxes of the NaCl and/or NaCl/KCl salts are normally used in reverberatory furnaces at a ratio of 0.6 to 0.8 flux to inert ingredient.
Using the flux with the above composition ratio leads to the production of a very viscous pastey slag which contains a high proportion of oxides and which can be physically removed from the furnace. However, this slag unfortunately contains a large quantity of metal which is not recovered in the furnace.